BaseTools: Fixed metafile parser issues

https://bugzilla.tianocore.org/show_bug.cgi?id=1406
This patch is going to fix the regressions that
is introduced by commit 2f818ed0fb

The internal array for storing the metadata info should be cached
so that the meta file is parsed only once in one build.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/Workspace/DscBuildData.py

@@ -881,7 +881,7 @@ class DscBuildData(PlatformBuildClassObject):
         return self._LibraryClasses
 
     def _ValidatePcd(self, PcdCName, TokenSpaceGuid, Setting, PcdType, LineNo):
-        if self._DecPcds is None:
+        if not self._DecPcds:
 
             FdfInfList = []
             if GlobalData.gFdfParser: